Switch circuit and composite high frequency elements

ABSTRACT

A switch circuit for switching the connection of the receiving or transmitting circuits of two communication systems to an antenna circuit, which comprises first and second switch means having two switching elements, the first switch means comprising a first inductance element between a port connected to the antenna circuit and a port connected to the second switch means; the second switch means comprising a second inductance element between a port connected to the first switch means and a port connected to a receiving circuit of a first communication system, and a transmission line constituting the first inductance element having a lower characteristic impedance than that of a transmission line constituting the second inductance element.

FIELD OF THE INVENTION

The present invention relates to a switch circuit for multi-band mobilephones usable in pluralities of communication systems, and ahigh-frequency composite part comprising it.

BACKGROUND OF THE INVENTION

As mobile equipments such as mobile phones, etc. have recently been usedby remarkably increasing numbers of users, their functions and serviceshave extremely improved. Portable wireless communication systems includeEGSM (extended global system for mobile communications) and GSM 1800(global system for mobile communications 1800) widely used mostly inEurope, GSM 1900 (global system for mobile communications 1900) widelyused in the U.S., and PDC (personal digital cellular system) used inJapan, etc. From the aspect of convenience by users and efficient use ofcommunications facilities, multi-band mobile phones such as dual-band ortriple-band mobile phones, which can utilize pluralities ofcommunication systems, have been developed.

A multi-band mobile phone can utilize pluralities of systems. As ahigh-frequency part used in such mobile phones, WO 00/55983 discloses ahigh-frequency switch module for switching transmitting circuits andreceiving circuits in different communication systems. Thishigh-frequency switch module comprises first and second filter circuitshaving different passbands, a switch circuit connected to the firstfilter circuit for switching a transmitting circuit and a receivingcircuit of a communication system A, and a switch circuit connected tothe second filter circuit for switching transmitting circuits ofcommunication systems B, C, a receiving circuit of the communicationsystem B and a receiving circuit of the communication system C.

The first and second filter circuits function as circuits for branchingtransmitting signals and receiving signals of the communication system Aand transmitting signals and receiving signals of the communicationsystems B, C. The switch circuit is a diode switch comprising a diodeand a transmission line as main elements, and any one of pluralities ofcommunication systems A, B, C is selected by controlling the diode in anON or OFF state by applying voltage from control circuits, therebyswitching the antenna and the transmitting circuits and receivingcircuits of the communication systems A, B, C.

Specific examples of the communication systems A, B, C are, forinstance, GSM, DCS 1800 and PCS. GSM corresponds to the above EGSM, DCS1800 corresponds to the above GSM 1800, and PCS corresponds to the aboveGSM 1900. Table 1 shows the transmitting and receiving frequencies ofeach communication system. TABLE 1 Communication Transmitting ReceivingFrequency System Frequency (MHz) (MHz) EGSM 880 to 915 925 to 960 GSM1800 1710 to 1785 1805 to 1880 GSM 1900 1850 to 1910 1930 to 1990

As shown in Table 1, the transmitting frequencies of GSM 1800 and GSM1900 are closer to each other than to that of EGSM. Accordingly, GSM1800 and GSM 1900 can use a common transmitting signal path in ahigh-frequency circuit. To switch the transmitting signals and receivingsignals of GSM 1800 and GSM 1900, a one-input, three-output switchcircuit comprising a common port for the transmitting signals of GSM1800 and GSM 1900, a receiving port of GSM 1800 and a receiving port ofGSM 1900 may be used.

As a one-input, three-output switch circuit, WO 00/55983 discloses adiode switch circuit comprising diodes and transmission lines as mainelements (FIG. 13). This diode switch circuit comprisescascade-connected, λ/4-switch circuits, to select a transmission mode ofGSM 1800/GSM 1900, a receiving mode of GSM 1800 and a receiving mode ofGSM 1900 as shown in Table 2 by controlling voltage applied to controlterminals VC2, VC3. TABLE 2 Mode VC2 VC3 GSM 1800 TX V+ 0 (Transmitting)GSM 1900 TX V+ 0 (Transmitting) GSM 1800 RX 0 0 (Receiving) GSM 1900 RX0 V+ (Receiving)

Zero voltage is applied to the control terminals VC2 and VC3 at the timeof receiving GSM 1800, to turn off diodes DP1, DP2, DD1 and DD2. Withthe diode DD1 in an OFF state, there is large impedance between aconnecting point IP2 and the transmitting circuit of GSM 1800/GSM 1900.With the diode DP1 in an OFF state, there is large impedance between aconnecting point IP3 and the receiving circuit of GSM 1900. Accordingly,the connecting point IP2 is connected to the receiving circuit of GSM1800 via two transmission lines ld3 and lp2.

Usually, the transmission line ld3 has such length that its resonancefrequency is within a frequency range (1710-1910 MHz) of transmittingsignals of GSM 1800 and GSM 1900, and the transmission lines lp2 hassuch length that its resonance frequency is within a frequency range(1930-1990 MHz) of a receiving signal of GSM 1900, with theircharacteristic impedance designed to be 50Ω.

However, intensive research has revealed that when the switch circuit isformed by strip lines, etc. in a multi-layer substrate, the reduction ofsize, particularly thickness, of the multi-layer substrate provides areceiving signal output port RX1 of GSM 1800 and a receiving signaloutput port RX2 of GSM 1900 with impedance of about 70-80Ω, larger than50Ω, by parasitic capacitance, etc., even though the characteristicimpedance of the transmission lines ld3 and lp2 is set at 50Ω, resultingin large loss in receiving signals from the output ports RX1, RX2. WO00/55983 does not recognize this problem, much less provides anysolutions.

OBJECTS OF THE INVENTION

Accordingly, an object of the present invention is to provide aone-input, three-output switch circuit comprising two cascade-connected,one-input, two-output switch circuits for switching the connection ofreceiving or transmitting circuits of two communication systems to anantenna circuit.

Another object of the present invention is to provide a smallhigh-frequency composite part with low transmission loss comprising suchswitch circuit in a multi-layer substrate.

DISCLOSURE OF THE INVENTION

The switch circuit of the present invention for switching the connectionof the receiving or transmitting circuits of two communication systemsto an antenna circuit comprises two switch means having switchingelements,

a first switch means comprising a first port connected to the antennacircuit, a second port connected to transmitting circuit of first andsecond communication systems, and a third port connected to a secondswitch means;

the second switch means comprising a fourth port connected to the thirdport, a fifth port connected to a receiving circuit of the firstcommunication system, and a sixth port connected to a receiving circuitof the second communication system;

a first switching element being disposed between the first port and thesecond port;

a first inductance element being disposed between the first port and thethird port;

a second switching element being disposed between the third port and aground;

a third switching element being disposed between the fourth port and thesixth port;

a second inductance element being disposed between the fourth port andthe fifth port; and

a fourth switching element being disposed between the fifth port and aground;

the third port being connected to the fourth port via a capacitanceelement; and

a transmission line constituting the first inductance element having alower characteristic impedance than that of a transmission lineconstituting the second inductance element.

With the above structure, the impedance of the fifth and sixth ports ofthe second switch means can be controlled, thereby achieving matchingwith a receiving circuit connected to each port and thus reducing thetransmission loss of a receiving signal.

When the first inductance element is formed by a transmission line suchas a strip line, etc. in a multi-layer substrate formed by laminatingceramic sheets, it is preferable to adjust characteristic impedance by agap between the transmission line and a ground and the width of thetransmission line. To provide the transmission line constituting thefirst inductance element with characteristic impedance lower than 50Ω,the transmission line preferably has an increased width. A widetransmission line has a small characteristic impedance and a reducedresistance, resulting in a further decreased transmission loss.

In the switch circuit of the present invention, the switching elementsmay be constituted by semiconductor elements such as field effecttransistors, bipolar transistors, PIN diodes, etc. The field effecttransistor increases or decreases impedance between its source and drainby a control voltage applied from a gate to permit or prohibit thepassing of high-frequency signals. The PIN diode increases or decreasesimpedance between its anode and cathode by a control voltage to permitor prohibit the passing of high-frequency signals. In any case, aswitching operation is carried out by changing the impedance.

The inductance elements may be transmission lines such as strip lineelectrodes, micro-strip line electrodes, etc, coils, chip inductors,etc. The capacitance elements may be laminated capacitors constituted bycapacitor electrodes, etc. These elements may be properly selecteddepending on applications.

In the switch circuit of the present invention, the first and secondinductance elements are preferably constituted by transmission lines,and a transmission line constituting the first inductance element ispreferably as long as ⅙ to 1/12 of the wavelength (λ) of signalstransmitted in the first communication system, and shorter than atransmission line constituting the second inductance element. With suchstructure, the transmission line can have small resistance and thusreduced transmission loss.

In the switch circuit of the present invention, a capacitance elementconnecting the third port to the fourth port between the first switchmeans and the second switch means preferably has capacitance of 10 pF orless. With the capacitance element having such capacitance, theimpedance of the fifth and sixth ports of the second switch means can beadjusted. Combined with the adjustment of the characteristic impedanceof the first inductance element, the impedance of the fifth and sixthports can be adjusted in a further wider range.

Impedance matching can be achieved between the transmission lineconstituting the first inductance element and the transmission lineconstituting the second inductance element by the capacitance element,whereby the first switch means is connected to the second switch meanswith good matching. The preferred capacitance of the capacitance elementis 2-7 pF.

The high-frequency composite part of the present invention comprisesswitching elements, capacitance elements and inductance elementsconstituting the switch circuit, which are mounted onto or contained ina multi-layer substrate formed by laminating pluralities of ceramicsheets, and connected through connecting means formed in the multi-layersubstrate, such as via-holes, connecting lines, etc.

In the high-frequency composite part of the present invention, at leastpart of the transmission line constituting the first inductance elementof the switch circuit is preferably wider than the transmission lineconstituting the second inductance element, thereby making the firstinductance element have lower characteristic impedance than that of thesecond inductance element. At least part of the transmission linesconstituting the first and second inductance elements are preferablyformed in a region sandwiched by ground electrodes in the multi-layersubstrate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing a high-frequency circuit comprising aswitch circuit according to one embodiment of the present invention;

FIG. 2 is a view showing an equivalent circuit of the switch circuitaccording to one embodiment of the present invention;

FIG. 3 is a view showing an equivalent circuit of a switch means used inthe switch circuit according to another embodiment of the presentinvention;

FIG. 4 is a view showing an equivalent circuit of a switch means used inthe switch circuit according to another embodiment of the presentinvention;

FIG. 5 is a graph showing transmission characteristics at the time ofreceiving in GSM 1800 in the switch circuit according to one embodimentof the present invention;

FIG. 6 is a view showing an equivalent circuit of the switch circuitaccording to another embodiment of the present invention;

FIG. 7 is a block diagram showing another example of a high-frequencycircuit comprising the switch circuit according to one embodiment of thepresent invention;

FIG. 8 is a block diagram showing a further example of a high-frequencycircuit comprising the switch circuit according to one embodiment of thepresent invention;

FIG. 9 is a plan view showing a high-frequency composite part comprisingthe switch circuit according to one embodiment of the present invention;

FIG. 10 is a perspective view showing a multi-layer substrate used inthe high-frequency composite part of FIG. 9;

FIG. 11 is an exploded plan view showing sheets constituting themulti-layer substrate used in the high-frequency composite part of FIG.9;

FIG. 12 is a view showing an equivalent circuit of the high-frequencycomposite part of FIG. 9; and

FIG. 13 is a view showing an equivalent circuit of a conventional switchcircuit.

BEST MODE FOR CARRYING OUT THE INVENTION

[1] Circuit Structure

FIG. 1 shows a high-frequency circuit comprising a switch circuit 10according to one embodiment of the present invention, and FIG. 2 showsan equivalent circuit of the switch circuit 10. For the simplificationof explanation, it is assumed that pluralities of communication systemsare GSM 1800 (transmitting frequency: 1710-1785 MHz, receivingfrequency: 1805-1880 MHz) as a first communication system f1, and GSM1900 (transmitting frequency: 1850-1910 MHz, receiving frequency:1930-1990 MHz) as a second communication system f2, though the presentinvention is of course not restricted thereto.

This switch circuit 10 comprising switching elements, inductanceelements and capacitance elements is constituted by a first switch means100 and a second switch means 105. The first switch means 100 comprisesa first port 100 a connected to an antenna circuit, a second port 100 bconnected to transmitting circuit of GSM 1800 and GSM 1900, and a thirdport 100 c connected to the second switch means 105. The second switchmeans 105 comprises a fourth port 105 a connected to the first switchmeans 100 via a capacitance element CP, a fifth port 105 b connected toa receiving circuit of GSM 1800, and a sixth port 105 c connected to areceiving circuit of GSM 1900.

The second switch means 105 comprises a transmission line lp2 as asecond inductance element between the fourth port 105 a and the fifthport 105 b, a diode DP2 as a fourth switching element between the fifthport 105 b and a ground, a capacitor CP1 between the fourth diode DP2and a ground, a diode DP1 as a third switching element between thefourth port 105 a and the sixth port 105 c, and a transmission line lp1or an inductor between the sixth port 105 c and a ground. A controlcircuit VC3 is connected between the diode DP2 and the capacitor CP1 viaan inductor LP and a resistor RP1. The transmission line lp2 is set at50Ω for matching with the characteristic impedance of an externalcircuit.

Disposed upstream of the second switch means 105 is the first switchmeans 100 for switching the transmitting circuit TX1, TX2 of GSM1800/GSM 1900 and the second switch means 105. The first switch means100 comprises two diodes DD1, DD2 and transmission lines ld2, ld3 (or aninductor in place of the transmission line ld2) as main elements.

Disposed between the first port 100 a and the second port 100 b is thediode DD1 as the first switching element, which has an anode connectedto the first port 100 a, and a cathode connected to the groundedtransmission line ld2. Connected between the first port 100 a and thethird port 100 c is a transmission line ld3 as the first inductanceelement, and a diode DD2 as the second switching element grounded via acapacitor cd4 is disposed on the side of the third port 100 c. A controlcircuit VC2 is connected between the diode DD2 and the capacitor cd4 viaan inductor LD and a resistor RD.

The characteristic impedance of the transmission line ld3 is 35-45Ω,lower than that of the transmission lines lp2. The first switch means100 and the second switch means 105 are connected with good matching bythe capacitance element CP. The length of a transmission lineconstituting the first inductance element is ⅙- 1/12 of the wavelength(λ) of a transmitting signal of GSM 1800, shorter than a transmissionline constituting the second inductance element.

A control logic of the control circuits VC2, VC3 for operating theswitch circuit may be the same as shown in Table 2. With the switchingelements controlled in an ON or OFF state by voltage applied from thecontrol circuits, transmission modes of GSM 1800/GSM 1900, a receivingmode of GSM 1800 or a receiving mode of GSM 1900 can be selected. Theoperation of the switch circuit will be explained in detail below.

(A) Receiving Mode of GSM 1800

When the receiving circuit RX1 of GSM 1800 is connected to the antennacircuit ANT, a zero voltage is applied from the control circuits VC2 andVC3 to hold the diodes DP1, DP2, DD1, DD2 in an OFF state. With thediode DD1 in an OFF state, there is large impedance between the firstport 100 a and the second port 100 b. With the diode DP1 in an OFFstate, there is large impedance between the fourth port 105 a and thesixth port 105 c. As a result, a receiving signal of GSM 1800 inputthrough the antenna is transmitted to a receiving circuit RX1 of GSM1800 via the transmission lines ld3, lp2 with low loss, without leakingto the transmitting circuit TX1, TX2 of GSM 1800/GSM 1900 and thereceiving circuit RX2 of GSM 1900.

(B) Receiving Mode of GSM 1900

When the receiving circuit RX2 of GSM 1900 is connected to the antennacircuit ANT, a zero voltage is applied from the control circuit VC2, anda positive voltage is applied from the control circuit VC3. The positivevoltage from the control circuit VC3 is applied to the second switchmeans 105 including the diodes DP1, DP2 with its DC component eliminatedby capacitors C20, C21, CP1, CP. As a result, the diodes DP1 and DP2 areturned on. With the diode DP1 in an ON state, there is low impedancebetween the fourth port 105 a and the sixth port 105 c. With the diodeDP2 and the capacitor CP1 in an ON state, the transmission line lp2 isgrounded at high frequencies, resonance occurs in a frequency band ofreceiving signal of GSM 1900, resulting in extremely large impedance ina receiving signal band of GSM 1900 when the fifth port 105 b is viewedfrom the fourth port 105 a. Further, with the diode DD1 in an OFF state,there is large impedance between the first port 100 a and the secondport 100 b. As a result, the receiving signal of GSM 1900 input throughthe antenna is transmitted to the receiving circuit RX2 of GSM 1900 withlow loss, without leaking to the transmitting circuit TX1, TX2 of GSM1800/GSM 1900 and the receiving circuit RX1 of GSM 1800.

(C) Transmission Mode of GSM 1800/GSM 1900

When the transmitting circuit TX1, TX2 of GSM 1800 and GSM 1900 areconnected to the antenna circuit ANT, a zero voltage is applied from thecontrol circuit VC3, and a positive voltage is applied from the controlcircuit VC2. The positive voltage from the control circuit VC2 isapplied to the first switch means 100 including the diodes DD1, DD2 withits DC component eliminated by capacitors C1, C2, cd4, CP. As a result,the diodes DD1, DD2 are turned on. With the diode DD1 in an ON state,there is small impedance between the second port 100 b and the firstport 100 a. Also, with the diode DD2 and the capacitor cd4 in an ONstate, the transmission line ld3 is grounded at high frequencies,resulting in resonance. As a result, impedance is large at the thirdport 100 c when viewed from the first port 100 a. Transmission signalsfrom the transmitting circuit of GSM 1800 and GSM 1900 are sent to theantenna terminal via the second filter circuit without leaking to thereceiving circuit. If the transmission line ld3 were short, there wouldnot be sufficiently large impedance when the third port 100 c is viewedfrom the first port 100 a, resulting in the leak of a transmittingsignal to the receiving circuit. Accordingly, the transmission line ld3is preferably as long as λ/12 or more.

FIG. 6 shows an equivalent circuit of the second switch means, in whichthe connection of the receiving circuits of GSM 1800 and GSM 1900 to thefifth and sixth ports is opposite to that in the first embodiment. Acontrol logic in this case is shown in Table 3. TABLE 3 Mode VC2 VC3 GSM1800 TX V+ 0 (Transmitting) GSM 1900 TX V+ 0 (Transmitting) GSM 1800 RX0 V+ (Receiving) GSM 1900 RX 0 0 (Receiving)

FIGS. 3 and 4 show equivalent circuits of other examples of the firstswitch means 100 and the second switch means 105. FIG. 3 shows anequivalent circuit in which a diode is used as a switching element, andFIG. 4 shows an equivalent circuit in which a transistor is used as aswitching element. Reference numerals assigned in both figures are thesame as in the first switch means.

This switch circuit can switch signal paths by voltage applied from thecontrol terminal VC2 like the above switch circuit. Incidentally, acontrol logic is different between a depression type and an enhancementtype in the transistors FET1, FET2. Used in the operation according tothe control logic shown in Table 2 is an enhancement-type FET, in whichimpedance between a source and a drain is low when voltage is applied toa gate. With such switch circuit, the same effects as above can beobtained.

[2] Laminate Structure

FIG. 7 shows a high-frequency composite part (multi-band antenna switchmodule) handling three communication systems, which comprising, inaddition to the switch circuits 10, 15 of the present invention, adiplexer circuit 300 and filter circuits 120, 125, 130, 135, 140 such aslowpass filters, bandpass filters, etc. in and on the multi-layersubstrate. FIG. 9 is its plan view, FIG. 10 is a perspective viewshowing the multi-layer substrate, FIG. 11 is a development showinglayers constituting the multi-layer substrate of FIG. 10, and FIG. 12 isa view showing an equivalent circuit of the high-frequency compositepart.

In this embodiment, inductance elements, capacitance elements andswitching elements for the switch circuit 10 are formed in themulti-layer substrate, together with inductance elements, capacitanceelements and switching elements constituting the diplexer circuit 300comprising first and second filter circuits 200, 210, lowpass filtercircuits 120, 125, and a switch circuit 15 in the high-frequency circuitshown in FIG. 7. The other filter circuits 130, 135, 140 are mountedonto the multi-layer substrate as SAW filters or FBAR filters.

Transmission lines as inductance elements are formed in the multi-layersubstrate, and diodes and high-capacitance capacitors that cannot becontained in the laminate are mounted as switching elements and chipcapacitors, respectively, onto the laminate, to constitute a one-chip,triple-band, high-frequency composite part.

The multi-layer substrate constituting this high-frequency compositepart can be produced by forming green sheets of alow-temperature-cofirable, dielectric ceramic as thick as 40 μm to 200μm, printing an Ag-based conductive paste on each green sheet to form adesired electrode pattern, and integrally laminating pluralities ofgreen sheets having the desired electrode patterns, and sintering theresultant laminate. Line electrodes constituting the transmission linesare preferably mostly as wide as 100 μm to 400 μm. Thelow-temperature-cofirable, dielectric ceramics may be, for instance, (a)ceramics comprising Al₂O₃ as a main component, and at least one of SiO₂,SrO, CaO, PbO, Na₂O and K₂O as an auxiliary component, (b) ceramicscomprising Al₂O₃ as a main component, and at least one of MgO, SiO₂ andGdO as an auxiliary component, (c) ceramics comprising Al₂O₃, SiO₂, SrO,Bi₂O₃ and TiO₂ as main components, etc.

Laminated green sheets are integrally compression-bonded, and sinteredat a temperature of about 900° C., to provide a multi-layer substratehaving a outer dimension of 6.7 mm×5.0 mm×1.0 mm, for instance. Thismulti-layer substrate is provided with terminal electrodes on its sidesurfaces. The terminal electrodes may be formed at properly selectedpositions on the bottom surface.

FIG. 11 shows the internal structure of the multi-layer substrate.Reference numerals of parts in FIG. 11 are in agreement with those ofthe corresponding parts in the equivalent circuit of FIG. 12. The firstand second transmission lines ld3, lp2 constituting inductance elementsin the switch circuit 10 of the present invention are formed in a regionsandwiched by a ground electrode G on a 10-th layer and a groundelectrode G on a 15-th layer, together with other transmission lineslp1, ld2 constituting the switch circuit 10 and transmission lines lg2,lg3 constituting the switch circuit 15 of SPDT. Electrode patternsconstituting the first and second transmission lines ld3, lp2 are formedon a 12-th layer to a 14-th layer and connected through via-holes (shownby black circles in the figure). The transmission lines are formed inhorizontally different regions such that they do not overlap in alamination direction. With such structure, interference can be preventedbetween electrode patterns constituting other circuit elements and thetransmission lines, resulting in improved isolation characteristics.Each transmission line can be made shorter by having a spiral shape.

With the electrode pattern constituting the first transmission line ld3wider than the electrode pattern constituting the second transmissionline lp2, the characteristic impedance of the first transmission lineld3 lower than that of the second transmission line lp2, and impedancematched by the capacitance element CP connecting the first switch means100 and the second switch means 105, the impedance of the receivingsignal output port RX1 of GSM 1800 and the receiving signal output portRX2 of GSM 1900 is adjusted to substantially 50Ω.

In this example, the first transmission line ld3 was as wide as 0.25 mm,about 2 times as wide as the second transmission line lp2, so that thecharacteristic impedance of ld3 was lower than that of lp2. Thecapacitance element CP was 3 pF.

FIG. 5 shows insertion loss at the time of the receiving mode of GSM1800. (a) indicates a case where both ld3 and lp2 had a characteristicimpedance of substantially 50Ω, and (b) indicates a case where thecharacteristic impedance of the transmission line ld3 was lower thanthat of the transmission lines lp2 (substantially 50Ω). In this example,the insertion loss was improved by about 0.2 dB. Insertion loss at thetime of the receiving mode of GSM 1900 was also improved by about 0.2dB. With such constitution, a high-frequency composite part havingexcellent isolation characteristics and transmission losscharacteristics can be obtained.

Though the specific examples of the switch circuit have been explainedabove, the switch circuit of the present invention is not restrictedthereto, and various modifications may be made without deviating fromthe scope of the present invention. Communication systems used in theswitch circuit of the present invention are not restricted to those inthe above embodiments. It is applicable, for instance, to a combinationof GSM 850 (transmitting frequency: 824-849 MHz, receiving frequency:869-894 MHz) and EGSM (transmitting frequency: 880-915 MHz, receivingfrequency: 925-960 MHz), and to high-frequency circuit blocks handlingfour different communication systems as shown in FIG. 8.

1. A switch circuit for switching the connection of receiving ortransmitting circuits of two communication systems to an antennacircuit, which comprises two switch means having switching elements, afirst switch means comprising a first port connected to said antennacircuit, a second port connected to transmitting circuit of first andsecond communication systems, and a third port connected to a secondswitch means; said second switch means comprising a fourth portconnected to said third port, a fifth port connected to a receivingcircuit of said first communication system, and a sixth port connectedto a receiving circuit of said second communication system; a firstswitching element being disposed between said first port and said secondport; a first inductance element being disposed between said first portand said third port; a second switching element being disposed betweensaid third port and a ground; a third switching element being disposedbetween said fourth port and said sixth port; a second inductanceelement being disposed between said fourth port and said fifth port; anda fourth switching element being disposed between said fifth port and aground; said third port being connected to said fourth port via acapacitance element; and a transmission line constituting said firstinductance element having a lower characteristic impedance than that ofa transmission line constituting said second inductance element.
 2. Theswitch circuit according to claim 1, wherein the transmission lineconstituting said first inductance element is as long as λ/6 to λ/12relative to the wavelength (λ) of signals transmitted in the firstcommunication system, and shorter than the transmission lineconstituting said second inductance element.
 3. The switch circuitaccording to claim 1, wherein a capacitance element connected betweensaid third port and said fourth port has capacitance of 10 pF or less.4. A high-frequency composite part comprising the switch circuit recitedin claim 1, wherein said switching elements, said capacitance elementsand said inductance elements are mounted onto or contained in amulti-layer substrate formed by laminating pluralities of ceramicsheets, and connected by connecting means formed in said multi-layersubstrate.
 5. The high-frequency composite part according to claim 4,wherein at least part of the transmission line constituting the firstinductance element of said switch circuit is wider than the transmissionline constituting the second inductance element.
 6. The high-frequencycomposite part according to claim 4, wherein at least part oftransmission lines constituting said first inductance element and saidsecond inductance element are formed in a region sandwiched by groundelectrodes in said multi-layer substrate.